Flat panel displays are widely used in a variety of applications, including computer displays. One suitable flat panel display is a field emission display. Field emission displays typically include a generally planar emitter substrate covered by a display screen. A surface of the emitter substrate has formed thereon an array of surface discontinuities or "emitters" projecting toward the display screen. In many cases, the emitters are conical projections integral to the substrate. Typically, contiguous groups of emitters are grouped into emitter sets in which the emitters in each emitter set are commonly connected.
The emitter sets are typically arranged in an array of columns and rows, and a conductive extraction grid is positioned above the emitters. All, or a portion, of the extraction grid is driven with a voltage of about 30-120 V. Each emitter set is then selectively activated by applying a voltage to the emitter set. The voltage differential between the extraction grid and the emitter sets produces an electric field extending from the extraction grid to the emitter set having a sufficient intensity to cause the emitters to emit electrons.
The display screen is mounted directly above the extraction grid. The display screen is formed from a glass panel coated with a transparent conductive material that forms an anode biased to about 1-2 kV. The anode attracts the emitted electrons, causing the electrons to pass through the extraction grid. A cathodoluminescent layer covers a surface of the anode facing the extraction grid so that the electrons strike the cathodoluminescent layer as they travel toward the 1-2 kV potential of the anode. The electrons striking the cathodoluminescent layer cause the cathodoluminescent layer to emit light at the impact site. Emitted light then passes through the anode and the glass panel where it is visible to a viewer. The light emitted from each of the areas thus becomes all or part of a picture element or "pixel."
The brightness of the light produced in response to the emitted electrons depends, in part, upon the rate at which electrons strike the cathodoluminescent layer. The light intensity of each pixel can thus be controlled by controlling the current available to the corresponding emitter set. To allow individual control of each of the pixels, the electric potential between each emitter set and the extraction grid is selectively controlled by a column signal and a row signal through corresponding drive circuitry. To create an image, the drive circuitry separately establishes current to each of the emitter sets.
In some embodiments, the voltage difference between the extraction grid and the emitter sets is controlled by setting the entire extraction grid to a single voltage and selectively coupling each emitter set to a reference potential, such as ground. One drawback of such an approach is that the drive circuitry for each of the emitter sets must respond to both the row signal and the column signal. This approach typically requires separate transistors or other current control elements for each of the row signal and the column signal such that each pixel requires at least a pair of current control elements.
Another approach to controlling the voltage differential between the extraction grid and the emitter sets is to divide the extraction grid into discrete sections each corresponding to a row of an array. The array of emitter sets is divided into discrete sections each corresponding to a column of the array. Each extraction grid row is connected to a respective row line while the emitters in each column are connected to each other and to a respective column line.
To activate this structure, one of the column lines is first grounded. Then, each of the row lines in the extraction grid is driven by a voltage corresponding to an image signal. To produce bright pixels, the row lines of the extraction grid are raised to a high voltage and to produce dim pixels, the row lines are held at a low voltage. The row lines arc therefore driven by rapidly switching, high analog voltages that require relatively expensive driver circuitry.
Another approach is to drive each of the row lines in the extraction grid with a constant magnitude voltage in response to the column signal and to drive column lines of the emitter substrate with analog voltages corresponding to the image signal. In this approach, the rows of the extraction grid are selectively biased at a constant grid voltage V.sub.G, one row at a time. During the time a row of the extraction grid is biased, each column line of the emitter substrate receives an analog column voltage corresponding to an image signal. The column line establishes the voltages of the emitter sets. The emitter set intersecting the biased row of the extraction grid will therefore emit light when the column line voltage is sufficiently below the voltage of the biased extraction grid row. The intensity of the emitter light will depend upon the voltage of the column line. If the column line voltage is very far below the grid voltage V.sub.G, the pixel will be bright. If the column line voltage is not very far below the grid voltage V.sub.G, the pixel will be dim. This approach, like the above-described approach involves switching relatively high voltages and requires relatively expensive drive circuitry.
One approach to reducing the cost of driver circuitry for driving column lines of liquid crystal displays is presented in U.S. Pat. No. 5,519,414, to Gold et al. and assigned to Off World Laboratories, Inc., which is incorporated herein by reference. In this approach, pulses applied to transmission lines constructively interfere to produce selected voltages at selected tap locations. The high voltages drive row lines coupled to the taps to establish voltages of emitter sets coupled to the column lines.
One difficulty in this approach is the effect of the taps on signal propagation in the transmission line. Each of the taps can be modeled as a shunting impedance coupled to the transmission line. Each tap therefore can cause reflections or loss of signal strength. For a line with many taps, the loss and reflections become very substantial, and taps located distant from the transmission line input receive very low voltage signals.
One approach to increasing the available signals at distant taps is to increase the voltage of the input signal. However, the increased signal can be excessive for taps located close to the signal input. Moreover, this approach becomes even more difficult for field emission displays, because voltage swings in field emission displays are typically much larger than for LEDs.